Browsing by Author "Scholten, J."
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- Ecosystem-level impact signals of groundwater borne continental nitrate transfer to the Ria Formosa lagoon by Submarine Groundwater Discharge (SGD) traced along the mixing gradient by a multi-indicator approachPublication . Rocha, C.; Veiga-Pires, C.; Wilson, J.; Aníbal, J.; Monteiro, José Paulo; Scholten, J.Recognition of the role played by SGD in the transfer of contaminants to near shore marine enviroments underscores the need for tools and aproaches that will facilitate regional assessments of its environmental impact.
- Estimating submarine groundwater discharge into the Ria Formosa lagoon, Portugal: Uncertainties in the lagoon-open ocean radium exchangePublication . Scholten, J.; Rocha, C.; Wilson, J.; Pham, M.; Veiga-Pires, C.; Aníbal, J.The Ria Formosa lagoon is the most important resource for the fishing, aquaculture and tourism industries of southern Portugal. The lagoon expands across an area of approximately 100 km²and about half of its area is intertidal, covered with muddy sand and saltmarshes. The lagoon is shallow (2 m average depth) with semidiurnal tides at amplitudes between 1.3 m and 3 m (during neap and spring tides respectively) causing a relatively rapid renewal of water through three inlets at time scales shorter than 5 days. The hinterland is characterized by intensive agriculture leading to elevated nitrate concentrations in coastal groundwaters. To quantify the amount of submarine groundwater discharge (SGD) and possible associated nutrient fluxes to the lagoon radium concentrations (223Ra, 224Ra, 228Ra, 226Ra) were measured during low and high spring tides in December 2009 and May 2010 using standard techniques (delayed coincidence counting, gamma spectrometry). A radium mass balance model accounting for all major sources and sinks of radium was employed to derive quantitative estimates of SGD. In order to avoid biases due to non-representative sampling, areal average radium concentrations were generated using the ArcGIS spatial analyst interpolation scheme. Radium exchange with the open ocean was the most significant parameter in the mass balance calculations. Three independent models were used to calculate this exchange flux: i) a tidal prism model using radium to calculate the return flow factor, ii) a hydrographic model providing outflow and inflow water fluxes for the Ria Formosa lagoon; and by combining these fluxes with the average concentrations measured at the inlet; iii) a model which estimates the exchange with the ocean based on the radium residence time calculated from the water exposure time in the lagoon. For each of these models we calculated SGD based on 223Ra, 224Ra and 226Ra mass balances the differences being at a maximum no greater than 35 %. However, the results from the models differed up to a factor of 2 with the tidal prism model producing the highest estimate and the residence time model the lowest estimates. We will discuss possible causes for the differences in the model results.
- Using Radium isotopes to evaluate the mixing timeline and relative age of waters in a leaky coastal lagoonPublication . Rocha, C.; Scholten, J.; Wilson, J.; Veiga-Pires, C.The Ria Formosa wetland system is classified as a leaky coastal lagoon and covers approximately 100 km2 of the South of Portugal, with roughly 50% being intertidal. Its hinterland is set in an arid region and on a coastal plain subject to intensive agriculture since the 50’s. In spite of high exchange coefficients with the coastal ocean (50 and 75% at neap and spring tides, respectively) and the annual total potential freshwater discharge from the hinterland a fraction of the daily tidal prism, worrying signs of eutrophication have been detected during the past couple of decades. These include fish- and clam-kill episodes, increased occurrence of nuisance algal blooms and substitution of native sea grass communities by macroalgae. Notwithstanding its critical importance for the evaluation of pollutant exposure period, the literature includes a wide range of estimates for the ‘residence time’ of waters within the lagoon (16 hours to 11 days, with an the apparent consensus falling within the 1-2 day interval) and this point is a clear obstacle for a correct environmental risk assessment, including management of the system. This lack of clarity is due in our view to two main factors: i) the lack of proper physical definition of the term ‘residence time’, with its consequent misuse and misapplication in context, a misconception that is unfortunately too common within the environmental community, and different concepts in the application of transport time scales ii) the geomorphological and hydraulic complexity of the system.. As part of ongoing research evaluating the role of Submarine Groundwater Discharge (SGD) as a loading vector for nutrients (especially Nitrate) into the lagoon, we use the radium quartet in combination with remote sensing and isotope mixing models to develop and discuss a mixing timeline for the system, We conclude that the average, whole-system residence time of waters within the lagoon is at least 4 days.